1. Field of the Invention
This invention relates to a system for making a heat-sensitive stencil master, and more particularly to a system for making a heat-sensitive stencil master in which the heat-sensitive stencil master is made by use of a thermal head comprising a number of heater elements.
2. Description of the Related Art
As a system for making a heat-sensitive stencil master, there has been known a system in which a thermal head having a number of heater elements is brought into contact with the thermoplastic film side of heat-sensitive stencil master material, thereby imagewise perforating the stencil master material.
FIG. 7 shows an example of such a conventional stencil master making system. In FIG. 7, a heat-sensitive stencil master material 1 is conveyed between a platen roller 3 and a thermal head 4 in the direction of arrow A by the platen roller 3 which is driven by an electric motor (not shown) while being pinched between a pair of driven rollers (conveyor rollers) 2. In this way, the thermoplastic film side 12 of the heat-sensitive stencil master material 1 is brought into contact with rectangular heater elements 40 of the thermal head 4, and by selectively energizing the heater elements 40 by a drive means (not shown), the thermoplastic film side 12 of the stencil master material 1 is perforated in an imagewise pattern.
FIG. 8 is an enlarged schematic plan view of the thermal head 4. In the thermal head 4, the heater elements 40 are arranged in a row in a main scanning direction, that is, a direction perpendicular to the direction of conveyance of the stencil master material 1 (sub-scanning direction). A pattern layer (electrode) 42 is connected to each side (in the sub-scanning direction) of each heater element 42 so that the heater elements 42 can be energized independently of each other.
When each of the heater elements 40 is energized and the temperature of the part of the thermoplastic film 12 in contact with the heater element 40 exceeds a shrinkage initiation temperature at which the thermoplastic film 12 begins to shrink, a fine perforation is first formed at a portion opposed to the center of the heater element 40 and is gradually enlarged outward, and when the heater element 40 is de-energized and the temperature of the part of the thermoplastic film 12 in contact with the heater element 40 lowers a shrinkage stop temperature at which the thermoplastic film 12 stops shrinking, the perforation is fixed.
In such a stencil master making system, the size of each heater elements 40 of the thermal head 4 is determined depending on the rate of feed of the stencil master material 1 in the sub-scanning direction and the resolution. In the conventional heat-sensitive stencil master making system, the size of each heater element 40 of the thermal head 4 is determined, for instance, to satisfy the following formula in order to make adequate the shape of the perforation and to prevent offset and/or run of ink due to excessive perforation.
B/Pb=xcex1xc3x97A/Pa wherein A represents the length of the heater element in the main scanning direction, B represents the length of the heater element in the sub-scanning direction, Pa represents the dot pitches in the main scanning direction, Pb represents the dot pitches in the sub-scanning direction and xcex1xe2x89xa71.0. Accordingly when the dot pitches in the main scanning direction and the dot pitches in the sub-scanning direction are equal to each other, the heater element 40 becomes longer in the sub-scanning direction than in the main scanning direction. See, for instance, Japanese Patent Publication Nos. 26838390 and 2732532.
Further it has been known that the size of the perforation is increased as the power supplied to the thermal head 4 is increased, is reduced as the heating time Tp of the thermal head 4 is shortened, and is increased in the sub-scanning direction as the heating time ratio xcex2 is increased, wherein the heating time ratio xcex2 is the ratio of the heating time Tp of the thermal head 4 to the speed of movement of the thermal head 4 T1 (line cycle) relatively to the stencil master material 1 (xcex2=Tp/T1).
Further it has been known that, in the case of the thermal head of the conventional heater element size, load on the thermal head is lightened and the durability of the thermal head is extended as the power supply is reduced and the heating time ratio xcex2 is increased. However, when the power supply is reduced and the heating time ratio xcex2 is increased, the perforations are enlarged in the sub-scanning direction and it becomes difficult to render the perforations discrete in the sub-scanning direction, which means that perforations are elongated in the sub-scanning direction and a proper stencil master cannot be obtained if the sub-scanning speed is increased.
Nowadays it is important to shorten the time required for printing. For this purpose, it is important to increase the perforating speed of the thermal head and make a stencil master at a higher speed. Specifically it is necessary to shorten the line cycle to not longer than 2.0 msec, e.g., 1.5 msec though it has been generally 2.5 msec.
However attempts to shorten the line cycle to 1.5 msec will encounter the following difficulties. That is, since the time for which power is supplied to the thermal head is shortened, the thermal head is not sufficiently heated and sufficiently large perforations cannot be obtained. This problem may be overcome by ensuring sufficient energy (power supplyxc3x97time) by increasing power supply to the thermal head. However this approach is disadvantageous in that the service life of the heater elements is shortened when the power supply to the thermal head is increased.
When the number by which the heater elements in the thermal head is divided in time division drive of the thermal head is reduced (e.g., when stencil master making is to be effected at a high speed, the number of division is reduced to 2 whereas the number of division is normally 4) so that power supply to each heater element is reduced and the heating time ratio xcex2 can be increased, load on the thermal head is lightened and durability of the thermal head is extended. However, such two-shift drive of the thermal head results in elongation of the heating time relatively to the dot pitches in the sub-scanning direction and the perforations arranged in the sub-scanning direction can be merged. When the perforations are merged, an excessive amount of ink can be transferred to the printing paper and problems such as offset, deterioration of image quality and the like can be caused.
Thus so long as the conventional thermal head in which each heater element is longer in the sub-scanning direction than in the main scanning direction is used, the heating time within which discrete perforations can be obtained is limited by the line cycle and it is difficult to form discrete perforations while driving the heater elements by reduced power supply so that durability of the thermal head is not shortened.
In view of the foregoing observations and description, the primary object of the present invention is to provide a stencil master making system in which a thermal head which can form discrete perforations even if the head heating time is increased.
Another object of the present invention is to provide stencil master making system which can form discrete perforations without shortening the service life of the thermal head even when the stencil master is to be made at a high speed.
In accordance with a first aspect of the present invention, there is provided a heat-sensitive stencil master making system comprising a thermal head having an array of a number of heater elements which extends in a main scanning direction substantially perpendicular to a sub-scanning direction in which the thermal head is moved relatively to heat-sensitive stencil master material when imagewise perforating the heat-sensitive stencil master material, wherein the improvement comprises that each of the heater elements is longer in the main scanning direction than in the sub-scanning direction.
In accordance with a second aspect of the present invention, there is provided a heat-sensitive stencil master making system comprising a thermal head having an array of a number of heater elements which extends in a main scanning direction substantially perpendicular to a sub-scanning direction in which the thermal head is moved relatively to heat-sensitive stencil master material when imagewise perforating the heat-sensitive stencil master material, wherein the improvement comprises that each of the heater elements satisfies the following formula (1),
B/Pb=xcex1xc3x97A/Pa (1 greater than xcex1xe2x89xa70.3)xe2x80x83xe2x80x83(1)
wherein A represents the length of the heater element in the main scanning direction, B represents the length of the heater element in the sub-scanning direction, Pa represents the dot pitches in the main scanning direction, and Pb represents the dot pitches in the sub-scanning direction.
It is preferred that the stencil master making system of the present invention be provided with a thermal head drive means which drives the thermal head so that the following formula (2) is satisfied,
0.25 less than xcex2 less than 1.0xe2x80x83xe2x80x83(2)
wherein xcex2 represents the heating time ratio Tp/T1 which is the ratio of the heating time Tp of the heater elements to the line cycle T1.
Further it is preferred that the stencil master making system of the present invention be provided with a sub-scanning means which conveys the stencil master material in the sub-scanning direction relatively to the thermal head at a speed v which satisfies the following formula (3)
V=Pb/T1xe2x80x83xe2x80x83(3)
Wherein Pb represents the dot pitches in the sub-scanning direction and T1 represents the line cycle which is not longer than 2.0 msec.
In the stencil master making system of the present invention, perforations which are discrete in the sub-scanning direction can be obtained even if the sub-scanning speed is increased so that resolution in the main scanning direction becomes equal to that in the sub-scanning direction since the heater elements in the thermal head is longer in the main scanning direction than in the sub-scanning direction.
Further even in the case where the resolution in the main scanning direction differs from that in the sub-scanning direction, the perforations can be discrete in the sub-scanning direction so long as formula (1) is satisfied.
Further when formula (2) and/or formula (3) is satisfied, a stencil master in which the perforations are of a proper size and discrete can be obtained even by high-speed stencil master making operation where the line cycle is short and it is difficult to elongate the heating time, and at the same time, since the heating time ratio xcex2 can be increased and power supply to the heater elements can be reduced, durability of the thermal head can be ensured.